It was released as a single through Sony Music Entertainment Japan on January 6, 2021, the same date as the duo's debut EP, The Book.

As an opening theme for the second season of the anime television series Beastars, "Kaibutsu" is about a carnivore trying to coexist with herbivores in a cruel world, based on Jibun no Mune ni Jibun no Mimi o Oshi Atete, written by the anime's writer Paru Itagaki.

On November 5, 2020, it was announced that Yoasobi would perform the opening theme for the second season of the anime television series Beastars, titled "Kaibutsu".

[6] "Kaibutsu" is an upbeat fast-paced song with a catchy chorus,[7] tells about a carnivore trying to coexist with herbivores in a cruel world.

[17] The music video "Kaibutsu" was premiered on January 13, 2021, directed by Rina Mitsuzumi, with 2D animation by Kaori Onishi and motion graphics by Jun Matsuda and Cafuu.

[7] The music video depicts the conflict and determination of the main character, Legoshi, that wears a sense of speed and darkness.

[21] The duo gave a television debut performance of the song at the 63rd Japan Record Awards on December 30, alongside "Yasashii Suisei", and "Moshi mo Inochi ga Egaketara".